DE-A-41 15 015 and DE-A-41 15 042 describe physically drying water-based coating compositions, in particular for the production of metallic coatings and for the production of plain coloured coatings. The metallic lacquers contain at least one binder based on polyurethanes containing carbonate groups and having a number average molecular weight (M.sub.n) of 70,000 to 500,000 and further polyurethanes having an M.sub.n of 20,000 to 60,000. Plain coloured coatings are produced by using binder mixtures based on polyurethanes containing carbonate groups and having a glass transition temperature (T.sub.g) of below 0.degree. C. and unsaturated reaction products of carboxy-functional polycondensation products with ethylenically unsaturated monomers having a glass transition temperature of above 0.degree. C.
Thanks to their ability to harden at room temperature or with slight heating, for example to temperatures of up to 60.degree. C., such physically drying coating compositions are particularly suitable for automotive repair lacquer coating. Once applied, they may be coated with transparent topcoat lacquers. Application may be performed wet-on-wet after briefly flashing off or once the base lacquer coat has dried. The clear lacquers used are customary transparent lacquers used in the automotive sector. These are in particular conventional formulations of acrylate/isocyanate-based two-component lacquers.
However, after overcoating with clear lacquer, the hardness of the base lacquer coat in such a multi-layer structure and consequently of the entire topcoat layer has been found to be inadequate. Particularly at elevated coat thicknesses (approx. 40 .mu.m), the base lacquer remains too soft. The gloss and water resistance achieved also remain unsatisfactory.
DE-A-37 31 652 describes a conventional two-component clear lacquer based on an acrylate resin containing hydroxyl, carboxyl and amino groups and a polyisocyanate crosslinking agent, which lacquer is in particular intended for application onto a metallic base coat lacquer. The acrylate resin is obtained by copolymerisation of hydroxyalkyl esters, .alpha., .beta.-unsaturated carboxylic acids with ethylenically unsaturated carboxylic acids and further olefinically unsaturated monomers. The resultant copolymer is further reacted with compounds containing one free isocyanate group and at least one tert.-amino group per molecule. Crosslinking is achieved at an NCO:OH ratio of 0.3:1 to 3.5:1. Only inadequate hardness in a multi-layer structure is obtained with these clear lacquers too.